Membrane potential of the transverse (T-) tubules in a striated muscle fiber controls the calcium permeability of the sarcoplasmic reticulum (SR). Information concerning T-tubule voltage changes is presumably transmitted to the SR at the unique morphological junctions (e.g. "triads") between these two membrane systems, but the mechanism of junctional transmission is unknown. The overall aim of this proposal is to determine the electrical properties of the T:SR junction in the most direct way technically possible, and to study the electrical signs of junctional transmission in the presence of as few extraneous, interfering electrical signals as possible. Junctional membrane will be electrically isolated in a voltage clamped, single muscle fiber by use of an extracellular micropipette for restricting measurement of membrane current density to a very small "patch" of membrane. Junctional areas can be identified by focally depolarizing the patch through the micropipette, thereby producing a local contraction. Both tubular (T:SR) and superficial (surface membrane: SR) junctions will be studies with this technique, and junctional electrical properties will be directly compared with those of nonjunctional surface membrane. Elucidating these properties is a necessary first step in understanding junctional transmission and Ca release, two extremely important and presently poorly understood aspects of skeletal muscle physiology, and will serve to guide further investigation of mechanisms involved in these processes in a fruitful direction.